GEOPHYSICAL DATA PROCESSING OF THE 2015 MICROEARTHQUAKE SWARM AT PLAINFIELD, CONNECTICUT

Starting in January 2015, several noticeable earthquakes were experienced in Plainfield, CT, constituting an earthquake swarm of 16 regionally recorded events. With magnitudes as high as 3.1, these events caused concern and incited intrigue in area residents as well as state and regional authorities. While seismic activity in the Northeastern U.S. is not uncommon, the combination of the magnitude, location, and number of events in this swarm is less than ordinary. Following the earliest events, four portable seismometers were stationed in the epicentral area, recording seismic activity continuously from January 13, 2015 through July 2015. In the analysis, all coincidental seismic signals within a short time window were associated from the four stations and the waveforms were plotted. Using waveform pattern matching, local earthquakes were discriminated from plots of coincidental noise or teleseismic events. Then, exact P-wave and S-wave arrival times were read from each station for every local event. These times were used to estimate the location and depth for each earthquake. The portable instruments have remained in place as local earthquake activity continued into July 2015, with a total number of 138 local events as of July 16, 2015. Observed changes in the shapes of the P waves and S waves from the same stations for different events suggest migrations of the earthquake sources. In New England, the relationship of modern earthquakes with known faults such as the Lake Char Fault, which is mapped in the immediate area of these earthquakes, is complex and not well understood. Although the spatial pattern of the locations of the earthquake swarm events does not delineate a single active structure, they do occur within the mapped Lake Char Fault zone at focal depths of about 1.5 km or less, suggesting that the Lake Char Fault zone should be further investigated for possible activity.